Swanne Gontharet - Academia.edu (original) (raw)

Papers by Swanne Gontharet

Logging data are measurements of physical properties of the formation surrounding a borehole, acq... more Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

&lt;p&gt;Heat flow is estimated at eight sites drilled during Integrated Ocean Dr... more &lt;p&gt;Heat flow is estimated at eight sites drilled during Integrated Ocean Drilling Program (IODP) Expedition 385 in the sedimented Guaymas Basin, Gulf of California. One of the expedition objects was designed to understand the thermal regime of the basin and to better understand heat transfer mechanisms from sill intrusions into organic-rich sediment. Sedimentation corrections are significant and increase basin values of heat flow values on average by 12% and range from 119 to 221 mW/m&lt;sup&gt;2&lt;/sup&gt; in the basin. Thermal analysis suggests that heat flow in the basin is distributed equally between conductive and advective heat transfer for plate ages older than 0.2 Ma. At Ringvent, Site U1547 a young sill intrusion is related to locally elevated heat flow displaying values between 257 and 1000 mW/m&lt;sup&gt;2&lt;/sup&gt;. Thermal analysis of the five holes drilled at Site U1547 suggests that the sill structure hosts an active hydrothermal system. Our study suggests that rapidly cooling intrusion led to discharge velocities between 15 &amp;#8211; 40 mm/yr and possibly recharge of the system may occur through normal faults. To be consistent with the heat output, we estimate the sill intrusion thickness to be ~240 m. The highly three-dimensional nature of the sill intrusion at Site U1547 and the question of its thickness add considerable complications which are currently investigated in complementary studies.&lt;/p&gt;

&lt;p&gt;&amp;#160; &amp;#160;During the International Ocean Disc... more &lt;p&gt;&amp;#160; &amp;#160;During the International Ocean Discovery Program (IODP) Expedition 385, diagenetic carbonates were recovered at different depths (from 10 to 534 mbsf) in six drilling sites (U1545, U1546, U1547, U1548, U1550 and U1552) distributed from the northern spreading segment of the Guaymas Basin (Gulf of California) to ~52 km away from the axial graben. As this basin is a site of active sea floor spreading, hemipelagic diatomaceous and organic-rich sediments rapidly accumulating at all these drilling sites are influenced by magmatic intrusions, at depths greater than 90 mbsf. The geothermal gradient observed ranges from 135 &amp;#176;C/km to 682 &amp;#176;C/km. Except in sites U1547 and U1548, both located close to a circular hydrothermal mound (called Ringvent), sills are in thermal equilibrium with the surrounding sediments.&lt;/p&gt;&lt;p&gt;&amp;#160; &amp;#160;Carbonate samples (over fifty) collected during this expedition are composed of fine-grained micritic sediments and weakly to strongly lithified carbonate concretions. The petrographic observations and XRD analyses show that stoichiometric dolomite is the dominant authigenic carbonate phase, with minor contribution of Fe-rich dolomite, and low- to high-Mg calcite. Centimeter-sized concretions composed of ankerite and Mg-calcite also occur in the sediments collected from the northern axial graben (site U1550) and a site with subsurface gas hydrates (U1552), respectively. Other authigenic minerals are often associated with these carbonates including pyrite, barite, anhydrite, zeolites and crystalline opal (opal-CT). The carbon isotopic compositions of the bulk carbonate from these samples exhibit large variations ranging from -28.9 to +12.0&amp;#8240; VPDB. Very low d&lt;sup&gt; 13&lt;/sup&gt;C values, only measured in a centimetre-seized concretion associated with cold methane seepage (site U1552), indicate that the bicarbonate used for carbonate precipitation is derived from anaerobic oxidation of methane (AOM) coupled with bacterial sulfate reduction (BSR). The very high d&lt;sup&gt;13&lt;/sup&gt;C values of diagenetic carbonates indicate the use of biogenic &lt;sup&gt;13&lt;/sup&gt;C-rich CO&lt;sub&gt;2&lt;/sub&gt; reservoirs related to active methanogenesis or other autotrophic microbial pathways. The oxygen isotopic compositions of these carbonates cover a large range from -5.3 to +4.0&amp;#8240; VPDB. The decreasing d&lt;sup&gt;18&lt;/sup&gt;O values with increasing depth reflect the effect of high geothermal gradients due to the close proximity of magmatic sills. &amp;#160;&lt;/p&gt;

Goldschmidt2021 abstracts, 2021

Marine Geology, 2009

... An Alltech Prevail Cyano column (150 mm × 2.1 mm, 3 mm) was used with hexane:propanol (99:1, ... more ... An Alltech Prevail Cyano column (150 mm × 2.1 mm, 3 mm) was used with hexane:propanol (99:1, 13 v) as a mobile phase. For the first 5 min, the flow rate of eluent was 0.2 ml min − 1 . In the following 45 min, the flow rate was used with a linear gradient to 1.8% propanol. ...

Deep Sea Research Part I: Oceanographic Research Papers, 2007

Scientific Data

Tidal marshes store large amounts of organic carbon in their soils. Field data quantifying soil o... more Tidal marshes store large amounts of organic carbon in their soils. Field data quantifying soil organic carbon (SOC) stocks provide an important resource for researchers, natural resource managers, and policy-makers working towards the protection, restoration, and valuation of these ecosystems. We collated a global dataset of tidal marsh soil organic carbon (MarSOC) from 99 studies that includes location, soil depth, site name, dry bulk density, SOC, and/or soil organic matter (SOM). The MarSOC dataset includes 17,454 data points from 2,329 unique locations, and 29 countries. We generated a general transfer function for the conversion of SOM to SOC. Using this data we estimated a median (± median absolute deviation) value of 79.2 ± 38.1 Mg SOC ha−1 in the top 30 cm and 231 ± 134 Mg SOC ha−1 in the top 1 m of tidal marsh soils globally. This data can serve as a basis for future work, and may contribute to incorporation of tidal marsh ecosystems into climate change mitigation and adap...

Journal of South American Earth Sciences, 2017

The North West migration of long and discontinuous mud banks along the French Guiana coast has be... more The North West migration of long and discontinuous mud banks along the French Guiana coast has been extensively studied during the past years, in particular with a large-scale vision, which consequently has integrated morpho and hydrodynamic data. The aims of the present paper were to use intrinsic sediment properties (grain-size, mineralogy, concentration, and cohesion) to (1) highlight the sedimentary conditions during the consolidation processes from fluid deposit to vegetation development, and (2) verify the apparent homogeneously derived sedimentary facies. Two intertidal transects, Macouria and Cayenne, were compared from the coast to offshore. Their altitude averages of 1 m and 2.8 m above mean sea level, respectively, were different enough to compare the influence of the hydrodynamic impact and emersion time on their sediment properties. The latter, i.e. grain size distribution, mineralogical content, mud concentration, and shear strength (cohesion), were determined from sampled surface sediments (first cm) and along sediment cores (20e30 cm depth) from each transect. A specific X-ray technique was applied to the whole core to differentiate clearly its thin layers. On both intertidal sites, the grain size dominated by the fine silt fraction (2e20 mm) and the bulk mineralogy characterized by five major minerals (quartz, feldspars, chlorite, illite, and kaolinite) appeared homogeneous along both transects and cores. In spite of this apparent uniformity of particle size and mineralogical parameters, as well as for visual observation along the core, high precision X-rays still showed a cyclic sedimentation at a micro-scale level. This cyclicity with intercalation of fine layers was related to distinct dynamic deposits marked by both tidal processes and hydrodynamic factors (swell propagation). The cohesion and concentration results were dependent on the topography, where high topography was characterized by sediments with high cohesion and concentration values, and vice versa. A comparison between these two parameters was done to define critical limits between soft and stiff muds, as well as unvegetated and colonized muds. The favorable intrinsic sedimentary properties for consolidation and colonization were also discussed according to the field observations and bibliographic data.

Rapid Communications in Mass Spectrometry, 2015

Impact of tidal cycles on the δ 13 C and δ 15 N of sedimentary OM

Journal of Coastal …, 2009

Recent work on Amazon-derived mud banks has revealed that desiccation processes can occur over el... more Recent work on Amazon-derived mud banks has revealed that desiccation processes can occur over elongated offshore mud bars that form on these banks. Once mud cracks appear there, opportunistic mangroves, i.e. Avicennia germinans, commence the colonization process. Through the advantage of buoyancy of the mangrove propagules which can be trapped by the mud cracks, propagules can establish sometimes 1-2 kilometres offshore of the adult colonies. In a few months, these pioneer colonies become new sources of propagules, promoting rapid ecosystem expansion. Thus, mud cracking may be seen as an important factor in mangrove resilience. A field experiment was conducted during the 2008 equinoctial spring tide in French Guiana. The study area is located on the landward face of an elongated mud bar. A digital elevation model (DEM) was computed from DGPS and high-resolution laser station data. Meteorological data including air temperature and humidity, wind speed and orientation, rainfall, solar radiation, temperatures of the mud surface and at a depth of 30-cm, were recorded. Additionally, the water content of the upper mud layer was monitored along a topographic transect. An installed camera took instant photographs every half hour. Preliminary results show that climatic conditions are not the main parameter controlling the desiccation process. Strengthening and consolidation of mud are mainly controlled by mud elevation (water loss by draining) and by the local tidal signal (determinant in spatial behaviour). The DEM enabled computation of the tidal frequency emersion signal, an important tool in analyzing the spatio-temporal patterns of mud cracking and mangrove colonization.

Chemical Geology, 2014

ABSTRACT The mobile French Guiana coast is a shoreface region downdrift of the Amazon River, wher... more ABSTRACT The mobile French Guiana coast is a shoreface region downdrift of the Amazon River, where enormous quantities of inorganic and organic materials are exchanged with the Atlantic Ocean. The rapid accumulation of these materials forms highly unstable shore-attached mud banks, which can be temporally emerged and then rapidly colonized and stabilized by microphytobenthos and opportunistic mangroves (i.e. Avicennia germinans). Mud banks are preferential sites for the accumulation and significant remineralization of organic matter (OM), due to intense erosion/deposition cycles and potential biological colonization. The distribution and sources of bulk sedimentary OM were characterized by elemental and isotopic analyses of four sediment cores, together with samples from three potential OM sources (mangrove plants, suspended particulate matter (SPM) and microphytobenthos), all collected from the landward face of the Macouria mud bank (French Guiana). Total organic carbon (TOC) and nitrogen (TN) concentrations in the sediment cores showed that OM sources were characterized by spatio-temporal variations in this mud bank. The relative contributions of mangrove plants, SPM and microphytobenthos were estimated using a three end-member mixing model based on [TN/TOC]atomic ratios and δ13C values. Sedimentary OM is mostly controlled by SPM associated with variable amounts of OM derived from mangrove plants and microphytobenthos. These variations could be explained by topography and bed elevation, which decrease submersion time and increase desiccation. Higher contributions of microphytobenthos are associated with black OM-rich laminae, identified in sedimentary cores and linked to temporal emersion phases of the mud bank, which favor the growth of benthic microalgae. This result is confirmed by the calculation of the average sediment accumulation rate (around 36.7 ± 14.8 cm y- 1) taking into account the emersion of the study site every spring tide (a fortnightly cycle). This value is within the range of the previous results from other mud banks in French Guiana, but is more than ten times greater than values generally obtained in other coastal contexts.